Yes, I have had the same on my 2011 +4. The anti squeal plates, ( as TBM correctly identified) are “stuck” to the back of the pads with some sort of goo. When mine slipped I removed them, cleaned up the back of the pads and applied some copperslip. Quck and easy job as long as you don't damage the H-clips when removing pins. (spares easily available from Rimmer Bros.)
I also know of a 2010 +4 where after about 8 years, the pads actually delaminated as Richard referred to.
